The invention relates to an apparatus for generating a light curtain comprising an image forming strip-like concave mirror and a light deflecting means arranged in the vicinity of the focal point of the concave mirror which periodically sweeps a bundled light beam coming from a radiation source over the concave mirror which extends in the scanning direction and/or receives a received light beam from the concave mirror and directs it to a photoelectric converter, wherein the radiation source or photoelectric converter and the light deflecting means on the one hand, and also the concave mirror and the beam exit or entry on the other hand, lie in different planes.
In apparatus of this kind an interruption of the scanning beam through a shadow effect caused by the deflecting unit is avoided by the fact that the beam source or photoelectric converter and the light deflecting means on the one hand, and the concave mirror and the beam outlet or inlet on the other hand, lie in two different planes. With an arrangement of this kind one can accordingly use not only the half aperture but rather the full aperture of the system. This arrangement in two different planes has however the consequence that the scanning beam no longer moves along a straight line on the image forming strip-like concave mirror but rather along a circular path. A circular path of this kind arises as result of the Z-angle shaped path of the beam and the fact that the point of origin of the light deflecting unit is not arranged at the center of curvature of the concave mirror, but rather in the vicinity of the focal point. As a consequence of the path of the beam along the circular track the beam is reflected at different angles in a plane perpendicular to the general scanning direction. Since an angular error is thus present the amount of the deviation becomes larger as the distance from the concave mirror increases.
In an apparatus of the initially named kind, as known from DE AS 26 22 113, this straight running error was admittedly partly compensated by a plane parallel plate. This correction plate however is a relatively large, precise and thus expensive component.
An optical scanning device is known from DE 35 14 302 A1 in which one of the image forming elements is formed as a spherical strip-like concave mirror.
Furthermore, a scanning device with a straight line light deflection is already known from DE OS 21 31 467 and has a mirror surface as a scanning element, with the mirror surface being helically wound and rotatable about the axis of the helix.